The present invention relates to a system for maintaining power consumption below a predetermined level and, more particularly, to an add/shed control for maintaining power consumption below a predetermined level where the loads can be added and/or shed in different sequences depending upon the prioritization level given to each load and upon the add/shed sequence assigned to each prioritization level.
Power utilities charge their customers according to a policy which is designed both to encourage energy conservation and to assess the cost for the extra power generating equipment which they must acquire and maintain to meet peak demands against those creating the peak demand. Accordingly, these power utilities will typically charge their customers at a first rate for power consumed below a first predetermined level, at a second rate for power consumed between the first predetermined level and a second predetermined level. If power consumption should exceed the second predetermined level, the utilities will charge what is in effect a penalty because they have had to acquire and maintain extra power generating equipment to meet these periods of unusually high demand. If all of the utilities customers could be encouraged to eliminate excessive usage of power, the power utilities would then not be required to provide the extra equipment to generate the extra power during these peak demand periods and would be able presumably to pass along these savings to its customers through lower charges. In order to encourage economies in power consumption by their customers, the public utilities, therefore, charge a "penalty" on those customers who create peak demands.
In order to avoid these peak demand charges imposed upon them by their power utilities, power consumers have employed automatic control systems which monitor power consumption within their facilities and will control the energization state of the power consuming loads within their facilities to maintain power consumption below some predetermined value in order to reduce the extra charges which would be imposed upon them during peak demand periods or intervals. These systems have typically taken the form of add/shed control systems which are designed to shed loads as power consumption approaches a predetermined level which is established by the customer. Alternatively, as power consumption begins to fall away from this predetermined level, previously shed loads can be added back on line so that they may be utilized by the customer.
Several different types of add/shed control systems have been proposed in the past. For example, the most common type of add/shed control system establishes a prioritized load order wherein the load having lowest priority will be shed first and the load having highest priority will be shed last. In such a system, if loads can be added back on line, the load having the highest priority will be added first and the load having the lowest priority will be added last.
Another add/shed system which is known adds/sheds loads on the basis of comfort fairness. That is, the loads associated with zones having conditions closest to setpoint will have the lowest priority and loads associated with zones having conditions the farthest away from setpoint will have the highest priority. In this system, the loads having lowest priority will be shed first and the loads having highest priority will be shed last on the basis that it is more fair to shed the loads associated with the zones being closes to the comfort range first.
These systems, however, are inflexible because rarely can all the loads within a building be made to fit within one of these types of add/shed routines. For example, it is more desirable to shed the air handling fan systems within the building according to a comfort fairness routine than according to a sequential routine because a sequential routine is a fixed routine and will not necessarily result in fair control of the air handling fan systems. On the other hand, the outdoor lights certainly would not be shed on the basis of comfort fairness and it may also be determined that the outdoor lights are all of equal priority so that they could not be prioritized according to a sequential routine. Thus, the outdoor lights may be shed rotationally so that the first shed will also be the first added.